They're twins, not doubles, and now researchers can use computer models to study them

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A twinned primary rainbow produced through computer simulation. The rainbow is split because of the interaction of light with two types of water drops: some smaller, spherical ones, and some larger water drops that become nonspherical. The different shapes cause light to leave the water drops in two different directions, which causes the rainbow to split into two arcs, a study found.

Double rainbows had their 15 minutes of fame on the Internet. Now get ready for their even more mysterious cousins: twinned rainbows. New research has suggested an explanation for these exotic shows of color.

Rainbows are known to form when sunlight interacts with tiny water drops in the atmosphere. As sunlight gets both reflected and refracted within the drops, it gets separated into its basic color components. Still, all the secrets of the more complex behavior of rainbows have long remained a puzzle.

The most common rainbow has a single arc. The less common double rainbow, which consists of two separate, concentric arcs, has inspired Internet memes. Triple and quadruple rainbows have even been spotted. Even rarer, however, is the twinned rainbow, where two arcs split from a single base rainbow.

"Everyone has seen rainbows, even double rainbows, and they continue to fascinate the scientific community," said researcher Wojciech Jarosz, a research scientist at Disney Research in Zürich. "Sometimes, when the conditions are just right, we can observe extremely exotic rainbows, such as a twinned rainbow. Until now, no one has really known why such rainbows occur."

Accidental discovery
Computer models of these natural wonders are helping Jarosz and his colleagues explain how twinned rainbows arise. The discovery was unintentional.

"In the course of us investigating and researching rainbows for graphics applications, we were really surprised to ultimately find out that rainbows were not fully understood," Jarosz told OurAmazingPlanet. "We are really excited that we were able to actually give some insight into a more purely scientific puzzle."

The international team of researchers studied the virtual rainbows for use in applications such as animated movies and video games, considering the physical shape of water drops and their complex interactions with light.

"Previous simulations have assumed that raindrops are spherical. While this can easily explain the rainbow and even the double rainbow, it cannot explain the twinned rainbow," Jarosz said.

Actual raindrops flatten as they fall because of air resistance. This flattening is more pronounced in larger drops. Such large water drops end up resembling the shape of hamburgers, earning them the name "burgeroids."

"It's not a very mathematical term, but we like to use it," said study researcher Henrik Wann Jensen, a computer graphics researcher at the University of California, San Diego.

The key to the mystery of twinned rainbows is the combination of different sizes of water drops falling from the sky.

"Sometimes two rain showers combine," Jarosz said in a statement. "When the two are composed of different sized raindrops, each set of raindrops produces slightly deformed rainbows, which combine to form the elusive twinned rainbow."

"We are the first to present an accurate simulation of twinned rainbows," said team member Iman Sadeghi, a software engineer at Google in Santa Monica.

Rainbow array
The software not only reproduced twinned rainbows seen in photographs, but a vast array of other kinds of rainbows as well.

"This goes beyond computer graphics," Jensen said in a statement. "We now have an almost complete picture of how rainbows form."

Although these simulations appear to explain twinned rainbows, "we have not validated this physically," Jarosz cautioned. "It would be nice to actually show that by manually producing showers with two raindrop sizes does in fact produce this effect."

The scientists were to detail their findings Wednesday at the SIGGRAPH 2012 conference in Los Angeles.